Turbochargers are a type of forced induction system. They deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. A smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, will reduce the mass and can reduce the aerodynamic frontal area of the vehicle.
Referring to FIG. 1, a turbocharger (10) uses the exhaust flow from the engine exhaust manifold to drive a turbine wheel (12), which is located in a turbine housing (14) to form a turbine stage (16). The energy extracted by turbine wheel (12) is translated into a rotating motion which then drives a compressor wheel (18), which is located in a compressor cover (20), to form a compressor stage (22). The compressor wheel (18) draws air into the turbocharger (10), compresses this air, and delivers it to the intake side of the engine.
Variable Geometry turbochargers typically use a plurality of rotatable vanes (24) to control the flow of exhaust gas, which impinges on the turbine wheel (12) and controls the power of the turbine stage (16). These vanes (24) also therefore control the pressure ratio generated by the compressor stage (22). In engines, which control the production of NOx by the use of High Pressure Exhaust Gas Recirculation (HP EGR) techniques, the function of the vanes (24) in a VTG also provides a means for controlling and generating exhaust back pressure.
An array of pivotable vanes (24) is located between a generally annular upper vane ring (UVR) (26) and a generally annular lower vane ring (LVR) (28). Each vane rotates on a pair of opposing axles (30) (FIG. 2), protruding from said vane (24) with the axles on a common axis. Each axle (30) is located in a respective aperture in the LVR (28) and a respective aperture in the UVR (30). The angular orientation of the UVR (26), relative to the LVR (28), is set such that the complementary apertures in the vane rings (26, 28) are concentric with the axis of the axles (30) of the vane (24), and the vane (24) is free to rotate about the axis (32) of the two axles (30), which is concentric with the now established centerline of the two apertures. Each axle (30) on the UVR side of the vane (24) protrudes through the UVR (26) and is affixed to a vane arm (34), which controls the rotational position of the vane (24) with respect to the vane rings (26, 28). Typically, there is a separate ring which controls all of the vane arms (34) in unison via small sliding blocks (48). This unison ring (50) is controlled by an actuator which is operatively connected to rotate the unison ring (50). The actuator is typically commanded by the engine electronic control unit (ECU). The assembly consisting of the plurality of vanes (24) and the two vane rings (26, 28) is typically known as the vane pack.
In a vane pack, the clearance between the rotatable vanes (24), more specifically between the cheeks (36) of the vanes (24) and the inner surfaces (38, 40) of the upper and lower vane rings (26, 28), is a major contributor to a loss of efficiency in both the control of exhaust gas allowed to impinge on the turbine wheel (12) and in the generation of backpressure upstream of the turbine wheel (12). The clearances between the vane side cheeks (36) and the complementary inner surfaces (38, 40) of the vane rings (26, 28) should be kept to a minimum to increase the efficiency of the vane pack.
However, minimizing such clearances can be difficult. Because the turbine housing (14) is not symmetrically round in a radial plane, and because the heat flux within the turbine housing (14) is also not symmetrical, the turbine housing (14) is subject to asymmetric stresses and asymmetric thermal deformation. Thermal deformation in the turbine housing (14) is transferred to the vane pack, which can cause the vane pack to wear, stick, or completely jam. Thus, the vane pack must be accurately placed and constrained within the turbine housing (14) in a manner which minimizes the transference of thermally induced distortion.
Thus, there is a need for a vane pack configuration that can minimize such concerns.